The present invention is directed to an aqueous processable, photoimageable element for use as a permanent coating for the protection of printed circuitry that is comprised of multiple layers of photoimageable compositions.
Photopolymerizable resist materials are known, for example, from U.S. Pat. Nos. 3,469,982 and 3,547,730, which describe a film resist with a sandwich structure in the form of a photopolymerizable layer between a cover sheet and a temporary support. This film resist can, for instance, be laminated on a copper base, exposed imagewise and developed with organic solvents or aqueous solutions, whereby a defined resist layer is formed. Typically the copper base for a printed circuit board is rigid, with limited flexibility of just a few degrees, such as the conventional copper-clad fiberglass epoxy laminates. More recently, printed circuits are being prepared on highly flexible film substrates to form electronic packages which may be folded and refolded one or more times to fit a specified configuration or a dynamic mechanical operation.
The defined resist layer thus produced can now be selectively etched, electroplated or treated with solder on the substrate. Particularly high demands are placed on photoresist films if they are used as permanent coatings that function as solder resists or masks. In this case, the developed, photopolymerized layer must withstand temperatures up to 300.degree. C. without degradation, loss of adhesion or accumulation of residues contained in or on the molten solder. With the advanced technology of today's printed circuit boards, it is important to have the capability to photoimage a solder mask. According to the current state of the art, such solder masks can be made by spraying, coating or calendaring liquid compositions on a substrate or also by laminating a dry film on a substrate.
Due to the adverse environmental impact of solvents, aqueous developable photopolymer systems with fast development are now preferred. The use of photopolymer resists with acid functions, primarily carboxyl functions, is known to impart aqueous processability. However, these groups are disadvantageous in many subsequent steps or events. In the case of photoresists, delamination of the resist is observed in alkali etching or gold plating and, in the case of solder masks, inadequate environmental resistance can occur. Modification of the carboxyl groups with melamine formaldehyde compounds to overcome the named disadvantages is known (EP 01 15 354 and U.S. Pat. No. 4,247,621).
The utilization of polymers containing carboxylic acid groups that are subsequently converted to less reactive and less moisture sensitive species is also known. U.S. Pat. No. 4,987,054 discloses a photo-polymerizable formulation yielding improved properties containing an acid copolymeric binder wherein a copolymer structural unit is the half acid/amide of a dicarboxylic acid. The disclosed formulations are used with conventional rigid printed circuit boards, processed with wholly aqueous alkaline solutions and are storage stable. European Patent Application EP 430,175 discloses a photopolymeric system similar to U.S. Pat. No. 4,987,054.
International Patent Application WO 93/17368 discloses an aqueous processable, photoimageable, permanent coating for printed circuits comprising (a) a cobinder consisting of a low molecular weight amic acid copolymer and a high molecular weight carboxylic acid-containing copolymer, (b) an acrylated urethane monomer component, (c) a photoinitiator system, and (d) a thermal crosslinking agent. The coating compositions of the present invention, however, do not contain an amic acid copolymer binder component and have reduced tack when compared to the coatings disclosed in WO 93/17368.
Solvent developable multiple layer photoresist films for use in the etching of printed circuit detail are also known. For example, U.S. Pat. No. 4,349,620 discloses a multilayer photosensitive film for use in the protection of printed circuits wherein the photosensitive layers consist of a plurality of layers having different properties, i.e. greater adhesion to a copper surface provided by one layer and greater toughness and reduced adhesion to a temporary support provided by the other layer.
U.S. Pat. No. 4,352,870 discloses a two-layer photoresist film wherein improved photo resolution is obtained through the resist layer closer to the substrate having a greater photosensitivity.
U.S. Pat. No. 4,506,004 discloses an improved printed circuit board obtained by means of a two-layer composite coating. An inner adhesive photopolymer layer is applied to the PCB in the liquid state displacing air from the PCB surface. A dry film solder mask is then temporarily adhered to the underside of a screen printing frame and applied onto the liquid layer prior to processing of the photopolymer layer. The laminated dry film solder mask is exposed through a phototransparency to harden the light struck dry film solder mask and light struck inner layer photopolymer, thereby cojoining the dry film solder mask, inner layer and PCB surface. A solvent exposure step removes unexposed dry film solder mask and unexposed inner layer photopolymer.
As the electronic industry is driven towards faster, more reliable and compact devices, there is an increasing need in the printed circuit field for a more flexible, permanent coating that can withstand typical manufacturing process conditions, such as molten solder, and continuously varying environmental conditions while maintaining its integrity. The capability of such a coating to stand up to varied conditions and, in addition, be photoimageable and aqueous processable would constitute an advancement of the art. Current protective coatings for polyimide flexible circuitry require mechanical punching or drilling before lamination in an overall costly, low productivity process. Particularly preferred is a photoimageable, aqueous processable, permanent coating for use with flexible circuitry that can be made with conventional, lower cost photoforming manufacturing processes that have higher resolution capability, and wherein the flexible circuits can be subjected to flexural stress and yet maintain functionality.